Pontoons have been proposed for attachment to the sides of an all-terrain vehicle to permit the same to be used in water. These pontoons are provided with vertical adjustment means operable in unison to lower the pontoons in operative position and to raise the same to an inoperative position: see U.S. Pat. No. 4,494,937 issued in 1985. These laterally-extending pontoons hamper road maneuverability, especially when the vehicle travels in woods, and the pontoons are prone to rapid wear and breakage, since they may strike big rocks, trees and the like. Once punctured, the can no longer be used as buoyant members. Another disadvantage of such a system is that the flotation level of each pontoon cannot be adjusted independently of the other, whereby the variable location of loads on the vehicle cannot be compensated for.